Mechanical Measurements of the Flux Lattice in the Heavy-Fermion Superconductor UPt3

Abstract

We report mechanical measurements of the flux lattice in U${\mathrm{Pt}}_3$. In the superconducting state the flux lattice that is formed in the sample when a magnetic field is applied contributes a restoring force and a dissipation term to a high-$Q$ mechanical oscillator, when the oscillator and sample are tilted with respect to the static magnetic field. In addition to the characteristic response for a type-II superconductor that gives $H_{c2\mathrm{}}\mathrm{}\left(T\right)\mathrm{}$, we observe an unexpected dissipation peak along two lines in the ($H, T$) plane within the superconducting state. The evidence suggests transitions between three different superconducting phases, and hence a complex phase diagram for U${\mathrm{Pt}}_3$ is proposed.